The main difference between the GAI and the GAII is the flow cell:it's larger in the GAII.
The main difference between GAII and GAIIx is that GAIIx can acquire data from 120 tiles per lane (only 100 tiles/lane in the GAII). It's due to a modification of the manifold and prism in the GAIIx.
The other difference is the amount of sequencing kits you can load at once on the machine. GAII has tanks than can accept up to 150 ml whereas GAII had to be reloaded every 36 cycles. Nevertheless, the read lengths are the same on the GAII and on the GAIIx.